One common application for thermal sensors is in thermal (infrared) detection devices such as night vision equipment. One class of thermal detection devices includes a focal plane array of infrared detector elements or thermal sensors coupled with an integrated circuit substrate with a corresponding array of contact pads between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements or pixels of the resulting thermal image.
In one embodiment, the thermal sensor includes a thermal sensitive element formed from a pyroelectric material that changes its electrical polarization and capacitance in response to thermal radiation, such as barium strontium titanate (BST). An infrared absorber and common electrode assembly is disposed on one side of the thermal sensitive element and comprises an optical coating overlaying a common electrode. A sensor signal electrode is disposed on the opposite side of each thermal sensitive element. The infrared absorber and common electrode assembly typically extends across the surface of the focal plane array and electrically couples each thermal sensitive element through the common electrode. Each infrared detector element or thermal sensor is defined, in part, by a portion of the infrared absorber and common electrode assembly and a respective sensor signal electrode, which constitute capacitive plates, and a thermal sensitive element, which constitutes a dielectric or insulator disposed between the capacitive plates.
A reduction in the thermal capacity of the thermal sensitive element may extend the performance limits of the associated thermal sensor. For a given value of thermal isolation between the thermal sensor and its environment, a reduction in the thermal capacity of the thermal sensitive element increases the temperature response of the thermal sensor to a given radiant flux. Several approaches use thinner thermal sensitive elements with reduced thermal capacity to improve the performance of a thermal detector.